The movable components of conventional zoom objectives are controlled by camming slots or the like in a rotatable drive member, usually a sleeve centered on the objective axis. The machining of the requisite camming grooves is complicated and relatively expensive; their coaction with respective cam followers gives rise to considerable frictional resistance, especially in certain positions in which one component may undergo a reversal of motion with reference to the other component, which results in rapid wear and consequent optical inaccuracies.